[Bug] Undefined variable in `config_server_interface.py `

* [Here](https://github.com/canonical/mongodb-operator/blob/6/edge/lib/charms/mongodb/v0/config_server_interface.py#L350) the access should be `self.charm.db_initialised = False`.
* Add a test for this case because a unit test would have caught this.

Open lib/charms/mongodb/v0/config_server_interface.py around line 350 and inspect the undefined-variable access. Add a unit test covering this case, then run the relevant unit-test suite. Done means the access uses the intended charm attribute and the regression test passes.

Written by the indexing model from the issue text.